Coronary heart disease (CHD) is commonly caused by atherosclerotic narrowing of the coronary arteries and is likely to produce angina pectoris, heart attacks or a combination. CHD caused 466,101 deaths in the USA in 1997 and is one of the leading causes of death in America today. Approximately, 12 million people alive today have a history of heart attack, angina pectoris or both. The break down for males and females is 49% and 51%, respectively. This year, an estimated 1.1 million Americans will have a new or recurrent coronary attack, and more than 40% of the people experiencing these attacks will die as a result. About 225,000 people a year die of coronary attack without being hospitalized. These are sudden deaths caused by cardiac arrest, usually resulting from ventricular fibrillation. More than 400,000 Americans and 800,000 patients world-wide undergo a non-surgical coronary artery interventional procedure each year. Although only introduced in the 1990s, in some laboratories intra-coronary stents are used in 90% of these patients.
One common type of coronary artery disease is atherosclerosis, which is a systemic inflammatory disease of the vessel wall that affects multiple arterial beds, such as aorta, carotid and peripheral arteries, and causes multiple coronary artery lesions and plaques. Atherosclerotic plaques typically include connective tissue, extracellular matrix (including collagen, proteoglycans, and fibronectin elastic fibers), lipid (crystalline cholesterol, cholesterol esters and phospholipids), and cells such as monocyte-derived macrophages, T lymphocytes, and smooth muscles cells. A wide range of plaques occurs pathologically with varying composition of these components.
A process called “positive remodeling” occurs early on during the development of atherosclerosis in coronary artery disease (CAD) where the lumen cross-sectional area (CSA) stays relatively normal because of the expansion of external elastic membrane and the enlargement of the outer CSA. However, as CAD progresses, there is no further increase in the external diameter of the external elastic membrane. Instead, the plaque begins to impinge into the lumen and decreases the lumen CSA in a process called “negative remodeling”.
Evidence shows that that a non-significant coronary atherosclerotic plaque (typically <50% stenosis) can rupture and produce myocardial infarct even before it produces significant lumen narrowing if the plaque has a particular composition. For example, a plaque with a high concentration of lipid and a thin fibrous cap may be easily sheared or ruptured and is referred to as a “vulnerable” plaque. In contrast, “white” plaques are less likely to rupture because the increased fibrous content over the lipid core provides stability (“stable” plaque). A large lipid core (typically >40%) rich in cholesterol is at a high risk for rupture and is considered a “vulnerable” plaque. In summary, plaque composition appears to determine the risk of acute coronary syndrome more so than the standard degree of stenosis because a higher lipid core is a basic characteristic of a higher risk plaque.
Conventionally, angiography has been used to visualize and characterize atherosclerotic plaque in coronary arteries. Because of the recent finding that plaque composition, rather than severity of stenosis, determines the risk for acute coronary syndromes, newer imaging modalities are required to distinguish between and determine the composition of “stable” and “vulnerable” plaques. Although a number of invasive and noninvasive imaging techniques are available to assess atherosclerotic vessels, most of the standard techniques identify luminal diameter, stenosis, wall thickness and plaque volume. To date, there is no standard method that can characterize plaque composition (e.g., lipid, fibrous, calcium, or thrombus) and therefore there is no routine and reliable method to identify the higher risk plaques.
Noninvasive techniques for evaluation of plaque composition include magnetic resonance imaging (MRI). However, MRI lacks the sufficient spatial resolution for characterization of the atherosclerotic lesion in the coronary vessel. Minimally invasive techniques for evaluation of plaque composition include intravascular ultrasound (IVUS), optical coherence tomography (OCT), raman and infrared spectroscopy. Thermography is also a catheter-based technique used to detect the vulnerable plaques on the basis of temperature difference caused by the inflammation in the plaque. Using the various catheter-based techniques requires a first step of advancement of an IVUS, OCT, or thermography catheter and then withdrawal of the catheter before coronary angioplasty thereby adding additional time and steps to the stent procedure. Furthermore, these devices require expensive machinery and parts to operate. This adds significant cost and time and more risk to the procedure.
Thus, a need exists in the art for an alternative to the conventional methods of determining plaque type. A further need exist for a reliable, accurate and minimally invasive system or technique of determining a plaque type or composition within a given blood vessel.